Sir Bernard Hogan-Howe said families of dead children whose identities were used would not be approached, as that could put undercover officers in danger. Photograph: John Stillwell/PA

Britain's most senior police officer has offered a general apology for the "morally repugnant" theft of dead children's identities by undercover spies who infiltrated political groups.

But Bernard Hogan-Howe, the Metropolitan police commissioner, has refused to tell any families if the identities of their children were stolen by the undercover officers. He said he wanted to protect the spies from being exposed.

In a report published on Tuesday, he admitted that at least 42 police spies stole the identity of children who had died before they were 14 years old.

But the total number of such spies could be far higher as he conceded that the technique could have been more widespread than initially believed.

The commissioner argued that the families could not be informed as it could lead to the exposure of the undercover officers sent to infiltrate the political groups.

"It was never intended or foreseen that any of the identities used would become public, or that any family would suffer hurt as a result. At the time this method of creating identities was in use, officers felt this was the safest option" he added.

His decision drew immediate criticism. Jenny Jones, a Green party member of the London Assembly, said: "This falls short of coming clean to all the families whose children's identities were harvested. In giving a blanket apology they have avoided the difficult task of apologising to real people."

The Met has sent letters of apology to 15 families whose children died young, but has neither confirmed nor denied whether identities were stolen.

One case concerned a suspected spy, deployed between 1999 and 2003, who allegedly stole the identity of Rod Richardson, who died two days after being born in 1973.

The family's lawyer, Jules Carey, said that Barbara Shaw, the mother of the dead boy, was taking legal action as she felt her complaint had been "swept under the carpet".

Carey said Hogan-Howe's apology was a PR exercise. He added: "The families of the dead children whose identities have been stolen by the undercover officers deserve better than this. They deserve an explanation, a personal apology. The harvesting of dead children's identities was only one manifestation of the rot at the heart of these undercover units."

Peter Francis, one of the spies who originally blew the whistle on the tactic, said the police should offer a personal apology to the families in the cases of spies whose identity had already been exposed. He agreed that the spies whose work remained secret should be protected.

The report, on Tuesday, was produced by Mick Creedon, the Derbyshire chief constable who is conducting an investigation into the activities of the undercover spies over 40 years.

Creedon revealed that the technique was used extensively as far back as 1976 and was authorised by senior police. He reported that the tactic became "an established practice that new officers were taught" within a covert special branch unit known as the special demonstration squad (SDS), which spied on political groups.

"This was not done by the officers in any underhand or salacious manner – it was what they were told to do," Creedon added.

One senior spy is quoted as saying the undercover officers "spent hours and hours … leafing through death registers in search of a name [they] could call his own".

"The genuine identities of the deceased children were blended with the officer's own biographical details," Creedon said.

The spies were issued with fake documents, such as passports and driving licences, to make their alter egos appear genuine in case suspicious activists started to investigate them.

The last time the tactic was used, according to Creedon, was 2003, by a spy working for a second covert unit – the national public order intelligence unit (NPOIU) – which infiltrated political campaigns.

Creedon said it was highly possible that the tactic was used by undercover officers in other units which infiltrated serious criminal gangs. "It would be a mistake to assume that the use of identities of dead children was solely within the SDS and the NPOIU."

He said that the use of the technique "however morally repugnant, should not detract from the [spies'] bravery".

Sign up for the Guardian Today

Our editors' picks for the day's top news and commentary delivered to your inbox each morning.